1. Field of the Invention
The present invention relates to an image pickup apparatus and more particularly to autofocus technique, for use in an image pickup apparatus, for automatically determining a focusing condition of a subject image.
2. Description of the Related Art
Image pickup apparatuses such as a digital still camera and a digital video camera having an image pickup device such as a charge coupled device (CCD) sensor or a complementary metal oxide semiconductor (CMOS) sensor are in practical use. A contrast detection system is known as an autofocus system applied to these electronic image pickup apparatuses.
The contrast detection system utilizes a phenomenon in which a captured image obtained by the image pickup device exhibits high contrast when a subject image formed on an imaging surface of the image pickup device is in an in-focus condition. Specifically, this system involves obtaining captured images in sequence, while effecting sequential changes in the imaging location of the subject image by shifting the position of a focus lens included in a shooting optical system or the position of the image pickup device. Then, this system involves calculating contrast evaluation values indicative of contrast of plural captured images obtained in sequence by the image pickup device, and judging as an in-focus location the position of the focus lens or the position of the image pickup device at which the contrast evaluation value is the maximum. The total value of high-frequency components in a spatial frequency spectrum of a captured image, for example, is used as the contrast evaluation value. A control algorithm for finding a maximum value of the contrast evaluation value for determination of the in-focus location, as mentioned above, is called “hill-climbing control.”
A conventional image pickup apparatus using the contrast detection system for execution of an autofocus process includes an evaluation value calculation circuit that is the circuit designed specifically for generation of the contrast evaluation value, and an image compression circuit provided independently of the evaluation value calculation circuit. The image compression circuit compresses the data amount of uncompressed digital captured image data to generate compressed image data in JPEG (Joint Photographic Experts Group) format, JPEG 2000 format, or other formats. Such a conventional image pickup apparatus first determines the in-focus location, using as an index the contrast evaluation value calculated by the evaluation value calculation circuit, and then uses the image compression circuit to compress captured image data obtained by shooting at the in-focus location, thereby to generate compressed image data for record.
Also, Japanese Patent Application Publication No. 2003-153068 (hereinafter, referred to as Patent Document 1) discloses an image pickup apparatus including an autofocus circuit (e.g., an AE/AF processing circuit as employed in Patent Document 1) that includes an evaluation value calculation circuit and that determines the in-focus location by finding the position of the focus lens at which the contrast evaluation value is the maximum, and an image compression circuit (e.g., a compression/expansion circuit as employed in Patent Document 1) that compresses each of plural digital captured image data generated by plural shooting operations at plural focus lens positions near the in-focus location and that outputs plural compressed image data to a memory. The image pickup apparatus disclosed in Patent Document 1 selects the compressed image data to be finally recorded from among the plural compressed image data retained in the memory, in view of the fact that a sharper image of a subject in focus becomes larger in data size after image compression. Specifically, this apparatus determines as the in-focus location the position of the focus lens at which the contrast evaluation value is the maximum, then executes the plural shooting operations at the plural focus lens positions near the in-focus location, then outputs the plural compressed image data to the memory, and finally selects the compressed image data of the largest data size as the compressed image data for record from among the plural compressed image data retained in the memory.